550 INSECTA. 



mode of nourishment of the larvae varies very greatly ; but their diet 

 consists mainly of vegetable substances, which stand in great abund- 

 ance at the disposal of the quickly-growing body. The larva usually 

 undergoes four or five, rarely a greater number of moults, and in 

 the course of its growth gradually assumes the form of the winged 

 insect, not in all cases by the direct transformation of parts already 

 present, but sometimes only after essential processes of new 

 formation. 



In this respect, however, there are considerable differences, the 

 extremes of which are represented in the Diptera by the genera 

 Coretkra and Musca. In the case of CoretJira, the larval segments 

 and the appendages of the head are transformed directly into the 

 corresponding parts of the perfect insect, while after the last larval 

 ecdysis the limbs and wings are formed from the imagined discs. 

 The imaginal discs are derived from the hypodermis of the larva. 

 The muscles of the abdomen and the other 

 systems of organs pass unaltered, or with 

 but little alteration, into those of the 

 adult animal. The thoracic muscles, on 

 the contrary, originate as fresh formations 

 from rows of cells already established in 

 the egg. With these slight changes, the 

 FIG. 458/-imago of piatygaxter active life of the pupa and the small de- 



(after Ganin). . 



velopment or the tat body are in necessary 



correlation. In Musca, on the contrary, the pupa of which is quiescent 

 and enclosed in a firm barrel-shaped membrane and contains a large fat 

 body, the body of the adult animal, with the exception of the abdomen, 

 arises by extensive transformations of the larva. The head and thorax 

 are developed from imaginal discs, which, already established in the 

 egg, become developed in the larva on the investing membrane of the 

 nerves or trachea?. It is not until the pupal stage that these discs 

 grow together, and give rise to the head and thorax. Every thoracic 

 segment is composed of two pairs of discs (a dorsal and a ventral), 

 the appendages of which represent the later wings and legs. All the 

 systems of organs of the larvae are said to undergo a disruption 

 during the protracted pupal stage as a result of the (recently, 

 however, contested) process of so-called histolysis, and are replaced 

 by new formations by aid of the fat body and the granular spheres 

 arising from the latter. 



When the larva has attained a certain size and degree of develop- 

 ment, i.e., when it is fully grown and provided with the food 



